This invention relates to battery charging apparatus and methods for charging a stack of multiple lithium ion battery cells.
As noted in my prior U.S. Pat. No. 5,773,959, which is incorporated by reference in this application, lithium polymer battery cells are unforgiving of overcharge. Overcharging damages the battery cell seriously. It is essential to avoid overcharging of a lithium polymer battery cell. The lithium polymer chemistry is such that it tends to self destruct above 4.2 volts, so it is desired to get all cells up as close to 4.1 volts as possible. This winds up to be a matching problem. By the time all four cells of a four cell stack, or all twenty cells in a twenty cell stack, get to 4.1 volts, the final balancing is desired to be done for 40 to 50 millivolts. This involves matching to about 1%. Matching to this degree can be difficult. A switched capacitor balancer can be associated with the stack to assist in matching the voltages. The charge can be moved from higher voltage cells to lower voltage cells in the stack by the action of the switched capacitor balancer, and the switched capacitor balancer can provide a relatively lossless balancing.
It is an important object of the present invention to charge a stack of multiple lithium ion battery cells by a combination of switch capacitance cell balancing and cell voltage monitoring to provide a charge cycle that starts with a nominally constant current charging and easily shifts to multi-cell constant voltage taper charging.